1. Field of the Invention
The present invention relates to the field of fiber optic transmission and more particularly to optical regeneration in fiber optic transmission systems. It applies in particular to RZ (return to zero) signal transmission systems, for example soliton signal transmission systems. The RZ signal corresponding to the transmission of a high logic value is referred to as a pulse or a xe2x80x9conexe2x80x9d. The absence of signal, which corresponds to the transmission of a low logic value, is referred to as a xe2x80x9czeroxe2x80x9d.
2. Description of the Prior Art
The transmission of soliton pulses (solitons) in the part of an optical fiber with abnormal dispersion is a phenomenon known in the art. Solitons are pulse signals with a sech2 shape. With this pulse shape, non-linearity in the corresponding part of the fiber compensates the dispersion of the optical signal. Modeling the transmission of solitons using the Schrxc3x6dinger non-linear equation is known in the art. Dispersion-managed soliton transmission systems have also been proposed. Such systems use a dispersion pattern which repeats periodically over thousands of kilometers; an abnormal dispersion (positive dispersion) fiber follows on from a normal dispersion (negative dispersion) fiber which compensates almost all the cumulative abnormal dispersion in the first type of fiber. The signals therefore propagate in fibers with high local dispersion but the overall average dispersion is very low. A transmission scheme of this kind significantly reduces the effects of jitter (because of the low average dispersion), significantly reduces the effects of collisions between channels (because of the high local dispersion), improves the signal-to-noise ratio and increases the spectral efficiency of the system. A system of the above kind is described in an article by N. J. Smith and N. J. Doran, Journal of Lightwave Technology, vol. 15, No. 10 (1997), p. 1808 et seq., for example.
Synchronous modulation of soliton signals by a clock signal (clock) to correct jitter has already been proposed; intensity modulation is described in an article by H. Kubota and M. Nakasawa, IEEE Journal of Quantum Electronics, vol. 29, No. 7 (1993), p. 2189 et seq., for example; an article by N. J. Smith and N. J. Doran, Optical Fiber Technology, 1, p. 218 (1995) proposes phase modulation.
A problem encountered in fiber optic transmission systems is the distortion to which optical signals are subjected when they are generated, transmitted or switched, more generally at the time of any optical processing in the transmission system. Optical regeneration by intensity modulation aims to solve this distortion problem by applying to each bit a signal of maximum intensity at the center of the time window of the bit and of low intensity at the edges of the time window.
However, intensity modulation does not provide an entirely satisfactory solution, in particular in respect of the xe2x80x9czerosxe2x80x9d of RZ signals. The noise in the xe2x80x9czerosxe2x80x9d, i.e. the noise in the time windows corresponding to a zero bit, or to a bit with no signal, is not completely eliminated, in particular in transoceanic transmission systems. There remain signals which are sometimes referred to as xe2x80x9cphantomxe2x80x9d peaks.
Furthermore, in standard optical regenerators, intensity modulation increases amplitude fluctuations and amplitude noise is stabilized by a narrow filter following regeneration. The quality of stabilization of the amplitude noise is good for a soliton signal transmission system using the standard Schrddinger equation propagation. However, it is a problem in dispersion-managed soliton signal transmission systems because this type of transmission is locally much less non-linear than Schrxc3x6dinger soliton transmission, the consequence of which is to reduce the efficiency of the filter.
The invention proposes a solution to this problem of noise in transmission xe2x80x9czerosxe2x80x9d. It eliminates noise by eliminating the xe2x80x9cphantomxe2x80x9d peaks.
The invention also reduces amplitude fluctuations in the xe2x80x9conesxe2x80x9d of RZ signals and in particular effectively stabilizes the amplitude of the xe2x80x9conesxe2x80x9d in dispersion-managed soliton signal transmission systems. This virtual suppression of amplitude noise in the xe2x80x9conesxe2x80x9d of RZ signals is accompanied by the possibility of dispensing with a narrow filter behind the regenerator of the invention. This opportunity not to associate any filter with the regenerator is one of the strengths of the invention, especially as a scheme of this kind is valid not only in standard Schrxc3x6dinger soliton signal transmission but also in dispersion-managed soliton signal propagation.
Thus the invention proposes a solution which eliminates xe2x80x9cphantomxe2x80x9d peaks in the xe2x80x9czerosxe2x80x9dxe2x80x94localized noise in the xe2x80x9czerosxe2x80x9dxe2x80x94and stabilizes the amplitude in the xe2x80x9conesxe2x80x9d of RZ signals, and in particular in Schrxc3x6dinger solitons or dispersion-managed solitons, without necessarily using a filter.
To be more precise, the invention proposes a regenerator for an RZ optical signal transmission system, including means for applying a relative time shift to the signals as a function of their intensity and an intensity modulator for modulating the shifted signals, wherein the shift between signals corresponding to a xe2x80x9czeroxe2x80x9d value and signals corresponding to a xe2x80x9conexe2x80x9d value is such that the modulation intensity for signals corresponding to a xe2x80x9czeroxe2x80x9d value is less than the modulation intensity for signals corresponding to a xe2x80x9conexe2x80x9d value.
In one embodiment the shift between signals corresponding to a xe2x80x9czeroxe2x80x9d value and signals corresponding to a xe2x80x9conexe2x80x9d value is such that the modulation intensity for signals corresponding to a xe2x80x9czeroxe2x80x9d value is minimum and the modulation intensity for signals corresponding to a xe2x80x9conexe2x80x9d value is maximum.
In another embodiment the shift between signals corresponding to a xe2x80x9czeroxe2x80x9d value and signals corresponding to a xe2x80x9conexe2x80x9d value is such that the modulation intensity for signals corresponding to a xe2x80x9czeroxe2x80x9d value is at least 6 dB less than the modulation intensity for signals corresponding to a xe2x80x9conexe2x80x9d value.
The modulation signal of the modulator is preferably a sinusoidal signal which has a period equal to the bit time and the shift between signals corresponding to a xe2x80x9czeroxe2x80x9d value and signals corresponding to a xe2x80x9conexe2x80x9d value is equal to half the bit time.
In another embodiment the regenerator further includes an amplifier whose output is connected to the shifting means.
The shifting means advantageously include a non-linear medium, for example a chalcogenide fiber.
In one embodiment the non-linear medium has an index of non-linearity n2 equal to n2=(Tbit/2)xc3x97(c/LNL)xc3x97(Seff/I1) where Tbit is the bit time, c is the velocity of light, LNL is the length of the non-linear medium, Seff is the effective cross section and I1 is the set point intensity of a signal corresponding to a xe2x80x9conexe2x80x9d value.
The non-linear medium can also be an optical cell.
The invention also proposes a regeneration method for an RZ optical signal transmission system, including the steps of time shifting the signals relative to each other as a function of their intensity and intensity modulating the shifted signals, wherein the relative time shift of signals corresponding to a xe2x80x9czeroxe2x80x9d value and signals corresponding to a xe2x80x9conexe2x80x9d value is such that the modulation intensity for signals corresponding to a xe2x80x9czeroxe2x80x9d value is less than the modulation intensity for signals corresponding to a xe2x80x9conexe2x80x9d value.
The shifting step advantageously includes shifting signals corresponding to a xe2x80x9czeroxe2x80x9d value relative to signals corresponding to a xe2x80x9conexe2x80x9d value so that the modulation intensity for signals corresponding to a xe2x80x9czeroxe2x80x9d value is minimum and the modulation intensity for signals corresponding to a xe2x80x9conexe2x80x9d value is maximum.
The shifting step can also include shifting signals corresponding to a xe2x80x9czeroxe2x80x9dvalue relative to signals corresponding to a xe2x80x9conexe2x80x9d value so that the modulation intensity for signals corresponding to a xe2x80x9czeroxe2x80x9d value is at least 6 dB less than the modulation intensity for signals corresponding-to a xe2x80x9conexe2x80x9d value.
In one embodiment the modulation step includes modulation by a sinusoidal signal whose period is equal to the bit time and the shifting step includes application of a shift equal to half the bit time between signals corresponding to a xe2x80x9czeroxe2x80x9d value and signals corresponding to a xe2x80x9conexe2x80x9d value.
The regenerator preferably further includes an amplification step before the shifting step.
In one embodiment the shifting step includes passing the signals through a non-linear medium, such as a non-linear medium in the form of an optical fiber, for example a chalcogenide fiber.
The non-linear medium advantageously has an index of non-linearity n2 equal to n2=(Tbit/2)xc3x97(c/LNL)xc3x97(Seff/I1)where Tbit is the bit time, c is the velocity of light, LNL is the length of the non-linear medium, Seff is the effective cross section and I1 is the set point intensity of a signal corresponding to a xe2x80x9conexe2x80x9d value.
The shifting step can also include passing the signals through a non-linear medium in the form of an optical cell.
Other features and advantages of the invention will become apparent on reading the following description of embodiments of the invention, which is given by way of example and with reference to the accompanying drawings.